Ctenophores

A beach-stranded ctenophore (perhaps Pleurobrachia)
found near the mouth of Humboldt Bay, CA, 2007.

Most people, many biologists included, get excited about mammals, birds, or other such furries and fuzzies. I’ve long been attracted to much more obscure groups of creatures. I think cyanobacteria are amazing; I love ferns and gymnosperms; I can’t get enough of kelps or red seaweeds. In college I was very interested in invertebrates, and thought for a time that I would do research on marine inverts once I started graduate school. In my professional work since I’ve steered towards photosynthetic organisms, but I still have a fondness for the inverts. One of the smallest phyla of invertebrates is the Ctenophora, also known as comb jellies or sea gooseberries. There are approximately 150 species globally. As a group they live completely in the ocean, with no freshwater representatives (Brusca and Brusca 1990).

Most comb jellies are pelagic organisms in the open ocean. As members of the plankton, ctenophores are largely passive drifters. However, they are also capable of some locomotion, powered by eight rows of fused cilia that line the outside of their gelatinous bodies. These rows of cilia, called ctenes, are one of the distinctive characteristics of the phylum. If your local aquarium has ctenophores on display, you are likely to be able to see the beautiful shimmering iridescence of the beating comb rows as the animals swim. A few ctenophores are benthic (bottom-dwelling).

One of my favorite ctenophores is Pleurobrachia, also known as the sea gooseberry. It is one of the model ctenophores that are introduced in invertebrate zoology courses. It has 8 rows of fused cilia like other members of its phylum. The body is nearly spherical and has an attractive radial symmetry (technically bi-lateral symmetry).

All ctenophores are believed to be predators, preying for example on marine zooplankton. In Pleurobrachia, there are two long tentacles emerging from its nearly spherical body. Specialized cells called colloblasts line the tentacles and are involved in prey capture. In essence, these cells burst and release adhesive materials on contact with the prey. The captured organisms are then drawn to the vicinity of the mouth when the tentacles retract towards the body. Some ctenophores also just passively capture food once it is caught in mucus on the outside of the body (talk about a free lunch!). 

Most ctenophores are hermaphrodites, producing eggs and sperm in the same individual. They have relatively simple life histories compared to many other invertebrates or marine algae. Fertilized eggs divide into an embryo and then into a larval phase known as a cidippid. The larval stage looks like Pleurobrachia. Like other “simple” organisms, ctenophores can also grow asexually – replacing even large portions of the body if damaged.

Beroid ctenophore. Credit: NURP, NOAA, archived here.

With translucent bodies and quasi-radial symmetry, many comb jellies resemble true jellyfish, but the latter are classified in class Scyphozoa of the phylum Cnidaria, and are a distinct evolutionary lineage of animals. Cnidaria possess stinging cells (pneumatocysts) with which they capture prey, but these specialized cells are lacking in the ctenophores. Though comb jellies and true jellies also both have simple nervous systems – usually characterized as a “nerve net” – recent research suggests their nervous systems are fairly distinct. The evolutionary relationships of ctenophores to other simple animal groups such as sponges (Porifera) and cnidarians has been a prominent research topic lately.

In late 2013, a team led by NIH researchers published the genome sequence of Mnemiopsis leidyi, an Atlantic Ocean ctenophore that is infamous for invading several Eurasian bodies of water and negatively impacting native food webs. Using DNA sequences from Mnemiopsis, Ryan et al. (2013) found evidence that ctenophores were the most ancient of major animal groups, branching from the animal tree of life before sponges, cnidarians and other animals. For a long time, sponges – which lack nerve cells and differentiated tissue layers – were believed to be the most primitive animal group. Last year, in another high-profile study, a large team of researchers compared the genome of Pleurobrachia and the transcriptome of additional ctenophores species with other animal groups (Moroz et al. 2014). Like Ryan et el. (2013), their findings also suggested that ctenophores are the most primitive major group of invertebrates.

Currently hypothesized relationships among animal groups including the ctenophores. Image from Figure 1f in Moroz et al. 2014, Copyright 2014 Macmillan Publishers Limited, under CC BY-NC-SA 3.0 license.

Both studies discussed the implications of this new understanding of ctenophore phylogeny for the evolution of nervous systems in animals as a whole. Because ctenophores have nerve and muscle cells (and sponges lack them), the question emerges about how many times nervous systems have evolved in animals. Moroz et al. (2014) found that ctenophore nervous systems are missing (or silence) many of the neurotransmitter molecules that are found in other animal groups, so they proposed that nervous systems may have evolved twice during the course of animal evolution: once for ctenophores, and once for cnidarians and more complex animals. The alternative hypothesis is that the animal nervous system evolved once in the common ancestor of all animals, but then was lost in sponges and another amoeboid-like group of invertebrates group known as the Placozoa. While this evolutionary question is far from settled (e.g., Ryan 2014), these are intriguing ideas pertinent to the early evolution of animal life some 600 million years ago. What is exciting is that biology continues to acquire new tools (in this case large-scale nucleic acid sequencing) to help address questions about the diversity of life that have been around for a long time!

Bibliography and citations

- Brusca RC and Brusca GJ. 1990. Invertebrates. Sinauer Associates, Inc., Sunderland, MA, 922 pp.

- Moroz LL et al. 2014. The ctenophore genome and the evolutionary origins of neural systems. Nature 510:109-114.

- Ryan JF et al. 2013. The genome of the ctenophore Mnemiopsis leidyi and its implications for cell type evolution. Science 342:1242591-1 to 8.

- Ryan JF. 2014. Did the ctenophore nervous system evolve independently? Zoology 117:225-226.